Risk of Obstetric Anal Sphincter Injury by Delivering Provider ^†

Introduction and Hypothesis: Obstetric anal sphincter injuries (OASIs) complicate 5.8% of vaginal deliveries. Our objective was to assess if the primary delivery provider, a nurse-midwife versus physician obstetrician, is associated with OASIs. Methods: We performed a secondary analysis of the Consortium of Safe Labor, a multicenter, retrospective cohort study. Included were nulliparous women with singleton, vaginal delivery at ≥37 weeks from 2002 to 2008. Women were excluded if delivery was complicated by shoulder dystocia or from sites without midwife deliveries. Student’s t-tests, chi-squared analysis, and Fisher’s exact test were used as appropriate. Multivariable logistic regression and propensity score-matching analyses were performed. Results: Of 228,668 births at 19 sites, 2735 births from 3 sites met the inclusion criteria: 1551 physician and 1184 midwife births. Of all births, 4.2% (n = 116) were complicated by OASIs. Physician patients were older, more often White, privately insured, with higher BMI, more medical co-morbidities, and labor inductions/augmentations. Midwife patients had higher fetal gestational age and infant birth weights (all p < 0.05). OASIs were more common in physician compared to midwife births (5.9% vs. 2.0%, p < 0.0001). This difference persisted in multivariable logistic regression. OASIs were 2.4 (95%CI 1.5–3.9) times more likely with physician delivery when controlling for maternal heart disease, episiotomy, increasing maternal age, decreasing maternal BMI, non-White race, and increasing birthweight. The AUC was 0.78. With propensity score matching, OASI rates remained higher amongst physician births (6.6% vs. 1.8%, p < 0.0001; aOR 3.8 (95%CI 2.0–7.1)). Conclusion: OASIs were more common in physician compared to midwife deliveries even when controlling for other associated factors.